WASHINGTON - With the U.S.-Russia relationship already strained, President Obama's decision to delay a possible military strike against Syria adds an unexpected twist to what was already shaping up to be an awkward G-20 summit hosted by Russian President Vladimir Putin this week in St. Petersburg.

Obama leaves Tuesday for Sweden, a stop added to his itinerary after he canceled a one-on-one meeting with Putin in Moscow ahead of the G-20, which begins Thursday.

The U.S. and Russia have long been at odds over the conflict in Syria as Russia, the only major patron of Syrian President Bashar Assad, has blocked any possibility of the United Nations Security Council mandating action against him in the 2½-year-old civil war.

Russia's blocking of the United Nations on Syria, along with the decision to grant former National Security Agency contractor Edward Snowden temporary asylum and a lack of recent progress between the two countries on other issues, led Obama last month to cancel the long-anticipated Moscow visit.

Obama announced over the weekend that he wants to take military action against the Assad regime in response to an alleged chemical attack Aug. 21, but first he wants congressional authorization. The G-20 meeting will offer the American president what is likely his last chance to muster international support from leaders of member countries for a potential strike.

Officially, the G-20 summit's focus is geared toward talks on a slew of global economic issues important to member countries. But Obama is likely to use sideline meetings with leaders to try to build a broader political coalition of countries willing to support U.S. military action in Syria, even if they aren't contributing militarily.

The White House says there are no plans for a one-on-one meeting between Obama and Putin on the sidelines of the summit, but the two will inevitably have some interaction during the course of the meetings.

In the days since Obama announced his desire to take military action against Syria, the chasm between the United States and Russia - at least rhetorically - has widened.

Over the weekend, the Obama administration and Putin traded barbs after Obama declared he wants to carry out a military strike.

Putin called it "nonsense" that Assad would authorize a chemical attack and urged Obama to consider whether a strike would have any impact to end the internecine violence or be worth the civilian casualties an American strike potentially could cost.

Secretary of State John Kerry expressed frustration that the Russians have turned a blind eye to evidence the United States has provided to demonstrate that the Syrian regime was responsible for chemical attacks against Syrian opposition this year, first confirmed by the U.S. intelligence community in June.

"They chose - I literally mean chose - not to believe it or to at least acknowledge publicly. I think this evidence is going to be overwhelming," Kerry said in an interview on ABC's This Week. "If the president of Russia chooses yet again to ignore it, that's his choice."

For the past two years, the Obama-Putin relationship has been "like watching a slow-moving train wreck," says Andrew Kuchins, a Russia expert at the Center for Strategic and International Studies in Washington.

The two leaders held glum expressions when they appeared for the cameras after their last meeting, the G-8 summit in Northern Ireland in June.

Last month, Obama described Putin as looking like the "bored kid in the back of the classroom" while acknowledging that the two have differences of opinion on some matters "and we're not going to be able to completely disguise them."

In perhaps Obama's most stinging rebuke, which came in a recent interview with The Tonight Show's Jay Leno, Obama lamented that Putin had a "Cold War mentality."

"One thing is clear to me, that this is the worst personal relationship between U.S. and Russian - perhaps even U.S. and Soviet - leaders in history," Kuchins said.

Despite the differences, Kerry said that the administration is looking at ways to internationalize efforts to secure Syria's chemical weapons cache and that he hopes Russia could play a role in that effort.

Though the relationship is undergoing particular strain because of differences on the Syria crisis and Russia granting asylum to Snowden, the White House notes that cooperation with Russia has not halted.

In recent weeks, Kerry and Defense Secretary Chuck Hagel held meetings in Washington with their Russian counterparts, and the two countries have held high-level meetings on the sticky issue of missile defense.

By delaying any military action until he has congressional approval, Obama has avoided having to defend the aftermath of a strike in a summit setting, notes Angela Stent, a Russia analyst at the Brookings Institution in Washington.

"Syria might still come up, but it won't dominate the G-20 agenda as it might have had there been a military strike," Stent said.